JPH03146649A - Production of alloying galvanized steel strip - Google Patents

Abstract

PURPOSE:To stably obtain a product of optimum degree of alloying by a simplified means by increasing the heating and temp.-rise rate of a steel strip in a heating zone of an alloying furnace to a specific value or above. CONSTITUTION:A steel strip 1 galvanized in a zinc bath 2 is subjected to alloying treatment by means of an alloying furnace 4 having a heating zone 5 and a holding zone 6, by which an alloying galvanized steel strip is obtained. In the above method, the heating and temp.-rise rate of the steel strip 1 in the heating zone 5 is regulated to >=2 deg.C/sec. By this method, the transition temp. of emissivity moves to the high-temp. side and the emissivity is held constant until a temp. of 420 deg.C at which plating is melted to the vicinity of 550 deg.C is reached. As a result, at the time of measuring the temp. of the steel strip 1 by means of a radiation thermometer 7 at the outlet of the heating zone 5, the true temp. of the steel strip 1 can be measured and the purpose can be accomplished. When the above heating and temp.-rise rate is lower than 2 deg.C/sec, the transition temp. of the emissivity becomes <=500 deg.C and a temp. at which alloying is performed and xsi-phase is formed cannot be reached.

Description

[Detailed description of the invention] <Industrial application field> The present invention relates to a method for manufacturing an alloyed galvanized steel strip.

<Prior art> In order to improve the paintability, coating adhesion, weldability, etc. of hot-dip galvanized steel strips, alloyed hot-dip galvanized steel strips in which the galvanized layer is alloyed with Fe-Zn are known. It is.

An example of a method for manufacturing an alloyed hot-dip galvanized steel strip will be explained with reference to FIG.
is heated to a predetermined temperature by passing it through.

The steel strip 1, which has been heated and cooled to a predetermined temperature in an annealing furnace, is then immersed in a zinc bath 2 at about 460° C., and molten zinc is deposited on its surface.

The steel f1 coated with molten zinc is pulled up from the zinc bath 2 in a substantially vertical direction, and the amount of zinc deposited is controlled by the wiping nozzle 3, and the steel f1 coated with molten zinc is pulled up from the zinc bath 2, and the amount of zinc deposited is controlled by the wiping nozzle 3. It is reheated to about 500° C. in the heat treatment furnace 4, subjected to alloying treatment, and sent to the next step.

As mentioned above, this alloying treatment is to form an Fe-Zn alloy layer by diffusing the iron of the steel strip 1 into the galvanized layer in order to improve the paintability of the galvanized steel strip. It is something.

Incidentally, such alloying is started from the moment m-l''l is immersed in the zinc bath 2, and is completed by reheating in the alloying furnace 4.

Here, the degree of progress of this alloying, that is, the degree of alloying, can be evaluated by visually determining the degree of hardening of the steel strip, or
Evaluation can be made by measuring the intensity of reflected light on the plated surface. Therefore, in a normal alloying hot-dip galvanizing apparatus, for example, an alloying degree measuring device (not shown) is placed in the alloying furnace 4 to measure the alloying degree of the steel strip, and the measured value is It is used to judge whether the product is acceptable or to adjust the reheating temperature in the alloying furnace 4.

For example, JP-A-57-185966 discloses a method for controlling the alloying position of a galvanized steel sheet in an alloying furnace by measuring the radiant energy of the plated surface of the steel sheet traveling in the alloying furnace. is disclosed.

Furthermore, for example, Japanese Patent Application Laid-Open No. 58-16061 discloses that the alloying position of a galvanized steel sheet in the alloying furnace 4 is controlled by measuring the intensity of reflected light from the plated surface of the steel sheet traveling inside the alloying furnace. A method is disclosed.

<Problem to be solved by the invention> By the way, in the production of alloyed galvanized steel strip, as alloying progresses, the emissivity of the plating surface decreases to approximately 0.1.
It is known that it varies greatly in the range of ~0.7.

This change in emissivity is due to the fact that the surface of the plating layer is in a molten zinc state (
The η phase (melting point 419°C) is alloyed to form the ζ phase (melting point 530°C).
℃), the temperature at which it becomes the δ phase (melting point 640℃) and solidifies (
(hereinafter referred to as transition temperature).

For this reason, when measuring the temperature of a steel plate in an alloying furnace using a radiation thermometer or the like in the above-mentioned Japanese Patent Application Laid-open No. 57-185966, since alloying is in progress at this thermometer position,
There was a problem in that the emissivity varied greatly depending on the measurement position and the sheet threading speed, making it impossible to measure the true steel sheet temperature.

Furthermore, the method disclosed in JP-A No. 58-16061 is a complicated method in which multiple reflected light intensity measurement units or one or two movable measurement units must be installed in the alloying furnace. .

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art described above and to provide a method for manufacturing an alloyed galvanized steel strip that can obtain an optimal degree of alloying by simple means.

<Means for solving the problem> When we investigated in the laboratory the changes in the emissivity of the plated surface during alloying of galvanized steel strips, we found that when the heating rate was increased, the emissivity decreased as shown in Figure 2. It was confirmed that the transition temperature of

In addition, when heating at a temperature increase rate of 2℃/second or more, the emissivity is constant at around 0.1 from 420℃ to around 550℃, where the plating melts, and as the temperature increase rate increases It was also found that the emissivity remains constant up to high temperatures. Furthermore, if the temperature increase rate is less than 2° C./sec, the transition temperature of the emissivity will be 500° C. or less, which is not preferable in terms of operation.

The present invention has been made based on these findings.

That is, according to a first aspect of the invention to achieve the above object, there is provided an alloyed galvanized steel strip in which a galvanized steel strip is alloyed in an alloying furnace having a heating zone and a holding zone. There is provided a method for producing an alloyed galvanized steel strip, characterized in that the heating rate of the steel strip in the heating zone is 2° C./second or more.

According to the second aspect of the present invention, in the manufacturing method, a radiation thermometer is provided at the outlet of the heating zone to measure the temperature of the steel strip, thereby controlling the heating conditions of the heating zone. A method of manufacturing characterized galvannealed steel strip is provided.

The present invention will be explained in more detail below.

The present invention is applicable to an alloying furnace 4 having a heating zone 5 and a holding zone 6 as shown in FIG.

That is, the steel strip 1 is heated and cooled to a predetermined temperature in an annealing furnace (not shown), then coated with molten zinc in a zinc bath 2, passed through a wiping nozzle 3, and then alloyed in a predetermined alloying furnace 4. It is processed under the specified processing conditions to become an alloyed galvanized steel strip.

The heating zone 5 is configured by direct heating, induction heating, or a combination thereof. The first aspect of the present invention
In this embodiment, the heating rate of the steel strip 1 in the heating zone 5 is set to 2° C./second or more. If the temperature increase rate is less than 2° C./sec, the emissivity transition temperature will be 500° C. or lower, and the temperature at which alloying will occur will not reach the ζ phase (melting point 530° C.).

There is no particular upper limit to the temperature increase rate. This is easily regulated by the thickness, width, threading speed, heating means, etc. of the steel strip, and is practically about 100° C./sec.

In order to make the temperature increase rate in the heating zone 5 a predetermined value of 2° C./sec or more, the furnace temperature or gas flow rate is adjusted when the heating zone 5 uses direct flame heating, or the plate passing rate of the input power when the heating zone 5 uses induction heating. It should be adjusted accordingly.

The holding f6 is provided following the heating zone 5, and holds the steel strip 1 heated to a predetermined temperature in the heating zone to a predetermined temperature (5
00 to 700° C.) for a predetermined period of time to form an alloyed galvanized steel strip.

The degree of alloying of the galvanized layer in the alloying furnace 4 depends on the alloying treatment conditions, that is, the heating temperature and the holding time at that temperature.
tz? It changes depending on the I4 degree and components such as P in the steel strip.

Figure 3 schematically shows an example of the relationship between heating temperature and holding time, but since the length of the holding zone is usually constant,
If the sheet passing speed changes, the holding time will also change, so it is necessary to change the heating temperature so that it falls within the appropriate range illustrated in FIG. 3. This appropriate range is determined in advance according to the type and proportion of alloying elements. In addition, in the case of an alloying furnace where the equipment length of the holding zone is variable and the holding time can be controlled constant even if the strip passing speed changes, it is necessary to
It is only necessary to change the heating temperature in response to changes in the n concentration, steel type, etc.

In this way, the steel strip 1 that has passed through the heating f5 is held in the holding band 6 for a predetermined period of time, and an alloyed galvanized steel strip that has been processed to an optimal degree of alloying can be manufactured.

Next, a second aspect of the present invention will be described, but since it is the same as the first aspect except that a radiation thermometer 7 is provided at the outlet of the heating element 5, a redundant explanation will be omitted.

The steel strip 1 immediately after plating is heated in the heating zone 5 at a temperature increase rate of 2° C./sec or more, and the temperature of the steel strip '1 is measured with the radiation thermometer 7 installed at the outlet of the heating element 5. Adjust the output of the heating zone 5 (furnace temperature or gas flow rate for direct flame heating, input power for induction heating) so that the temperature reaches the preset target temperature, and then adjust the output to the appropriate range as shown in Figure 3. By holding the steel plate temperature at 'lf6 for a predetermined period of time, a galvanized steel strip with an optimum degree of alloying can be manufactured.

That is, by setting the temperature increase rate in the heating zone 5 to 2°C/second or more, the emissivity temperature becomes high temperature, and the emissivity is 0.1 from 420°C, where the plating melts, to around 550°C. This is because the true temperature of the steel strip 1 at the exit of the heating IF 5 can be measured since it is constant in the vicinity.

At this time, when measuring with the radiation thermometer 7, the emissivity of the radiation thermometer 7 must be set.

Since this emissivity differs depending on the detection wavelength of the radiation thermometer 7, etc., the relationship between heating temperature and holding time allows you to set an accurate emissivity by calibrating it using the steel plate temperature measured with a contact thermometer, etc. in actual equipment. As mentioned above.

Further, it is also possible to control the degree of alloying by calculating the optimal heating temperature using a process computer from the relationship between the heating temperature and the holding time and giving it as a target value.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) Using an alloying furnace as shown in Fig. 1, galvanized steel
f (steel type: low carbon steel, thickness: 0.5 to 1.2 mm, width 8
00 to 1300 mm), the heating rate was 5 to b.Measuring the plate temperature at the outlet.Measuring the plate temperature with a radiation thermometer with a wavelength of 2 μm and a contact thermometer, the set emissivity of the radiation thermometer ε =
0.12, it was possible to measure the plate temperature in the range of 440 to 580°C with an accuracy of ±5°C.

The length of the holding band 5 is 25 m, and the threading speed is 80 m/
At min, the retention time was 18.8 seconds, at 100 m/
For min, it is 15 seconds.

Therefore, by applying the conditions schematically shown in Figure 3,
When threading at a rate of m/min, the heating temperature was set at 490° C., and the heating conditions were feedback-controlled to obtain a product with an optimal degree of alloying.

Further, when the sheet passing speed was changed to 80 m/min, the heating temperature was changed to 470° C., and a product with the optimum degree of alloying was obtained.

Practically speaking, it is also possible to determine the heating temperature that will result in optimal alloying using an alloying degree needle installed online, and to perform feedback control to achieve that temperature.

<Effects of the Invention> Since the present invention is configured as explained above, an alloyed galvanized wI band having an optimum degree of alloying can be obtained by simply performing rapid heating at 2° C./second or more in the heating zone. can be manufactured.

In addition, by performing rapid heating at 2° C./second or more in the heating zone and installing a radiation thermometer at the exit of the heating zone, it became possible to measure the true temperature.

In addition, since the heating temperature and holding time can be clearly defined, the heating temperature can be changed according to the holding time corresponding to the threading speed, and the heating amount can be controlled stably and optimally by feedback control of the radiation thermometer output. It became possible to obtain alloyed products.

Furthermore, by having a process computer calculate these operational parameters, it is possible to perform alloying processing fully automatically.

[Brief explanation of the drawing]

FIG. 1 is a layout diagram showing one example of an alloying furnace for carrying out the present invention. FIG. 2 is a graph showing the relationship between the emissivity of the plating surface and the vA bath temperature. FIG. 3 is a graph schematically showing the relationship between holding time and heating temperature. Explanation of symbols 1... Steel strip, 2... Zinc bath, 3... Wiping nozzle, 4... Alloying furnace, 5... Heating zone, 6... Holding zone, 7... Radiation thermometer FIG, 2 i initial temperature C)

Claims (2)

[Claims] (1) A method for producing an alloyed galvanized steel strip, in which a galvanized steel strip is alloyed in an alloying furnace having a heating zone and a holding zone, comprising: heating temperature increase rate of the steel strip in the heating zone; A method for producing an alloyed galvanized steel strip, characterized in that the temperature is 2° C./second or more. (2) In the method for producing an alloyed galvanized steel strip according to claim 1, the heating conditions of the heating zone are controlled by providing a radiation thermometer at the outlet of the heating zone to measure the temperature of the steel strip. A method for producing an alloyed galvanized steel strip characterized by: